1. Field of the Invention
This invention relates to the field of circuit design. In particular, the invention relates to a method and apparatus for securing data used to configure a programmable logic device.
2. Background Information
Programmable Logic Devices (PLDs) are a class of devices that allow a user to program a device to perform the function of a particular circuit. Examples of PLDs are FPGAs (Field Programmable Gate Arrays) and EPLDs (Erasable Programmable Logic Devices).
To use a PLD, a user captures a circuit design using any of several design capture tools. The user then uses software tools to convert the captured design into a device specific bitwise representation. The bitwise representation is stored in a storage device, such as an EPROM. Upon startup, the storage device supplies the bitwise representation to the PLD, thereby enabling the PLD to perform the function of the circuit design. The PLD, having read in the bitwise representation, then performs the function of the circuit design.
By the time the bitwise representation is created, significant amounts of time and effort have been expended. To encourage individuals and companies to continue to invest in the research and development of new circuit designs, it is desirable to provide some method of protecting the circuit designs from illegal copying and use.
To make an illegal copy of the circuit design, as implemented in the programmable logic device, one need only make a copy of the bitwise representation stored in the storage device. The copied bitwise representation can then be illegally used with other programmable logic devices. Therefore, it is desirable to make it more difficult to copy the bitwise representation of the circuit design.
Additionally, some types of PLDs support multiple configuration modes. For example, the XC4000.TM. series FPGAs, available from Xilinx, Inc. of San Jose, Calif., supports multiple configuration modes. The 1994 Xilinx Data Book, page 2-25 through page 2-46, describes the unsecured configuration modes for the XC4000.TM. FPGA product family. Therefore, it is desirable to have secure configuration of PLDs that have multiple configuration modes. Of course no system can be absolutely secure from all potential unauthorized access, therefore, the term "secure" is used to mean more secure than systems without any security.
Some PLDs can be chained together for the purpose of configuration. After one PLD is configured, the configuration data is passed to the next PLD in the chain. Therefore, it is desirable to support the secured configuration of multiple chained PLDs.